The invention relates to an on-board electrical system for an automobile. The on-board electrical system includes a vehicle battery, an electrical load and an energy store with a positive and a negative terminal. The invention also relates to an automobile with such on-board electrical system and to a method for operating an electrical load in an on-board electrical system of an automobile.
Modern automobiles have progressively more electrical loads consuming high power. Such electrical loads are used to replace mechanical or hydraulic systems in order to achieve lower fuel consumption with improved functionality. In particular, these are electric motors which are used, for example, for the steering system as well as in a braking system (for example the ESP, the “Electronic Stability Program”). The current consumption of these electrical loads is not constant, because they are switched in only when needed. A starter for an internal combustion engine represents such an electrical load, whose current consumption has very strong variations, namely depending on the rotation speed.
In the simplest situation, the on-board electrical system of an automobile includes a vehicle battery, a generator and a plurality of energy loads. When the engine is running, the generator supplies an electrical voltage used to power the loads and to charge the vehicle battery. The power supplied by the generator can also be adapted by a controller to the respective instantaneous current demand of the electrical load. However, the modern electrical loads strain the on-board electrical system with high pulsed currents. Presently used generators are too sluggish to supply these pulsed currents or to quickly increase or decrease the voltage. The on-board electrical system voltage is therefore mainly stabilized by the vehicle battery, and the quality of the on-board electrical system voltage is determined by the internal resistance of the vehicle battery. The on-board electrical system voltage can drop by several volts under the high pulsed currents, so that the functionality of sensitive loads may be temporally impaired. Such behavior is quite problematic, in particular with the novel start/stop systems in automobiles.
Different systems for reducing the voltage drops and for protecting sensitive loads were developed in the past. Most of these systems are based on double-layer capacitors or batteries which are used as additional energy stores in the on-board electrical system. In many conventional systems, the additional energy store is connected in parallel with the vehicle battery; this parallel connection reduces the total impedance so that the voltage drop of the on-board electrical system becomes smaller. Such on-board electrical systems are disclosed, for example, in the published documents DE 10 2005 015 995 A1 and DE 10 2007 026 164 A1.
An additional energy store implemented as a DC converter can also be added, which is described in the published documents WO 02/066293 A1 and DE 198 59 036 A1.
It is also known in the art to supply the sensitive loads directly from the additional energy store and to thereby decouple the load from the high-power loads. Such approach is described, for example, in Robert Bosch GmbH, “Autoelektrik, Autoelektronik, Systeme and Komponenten”, 4th edition, Vieweg Verlag, Wiesbaden, ISBN 3-528-13872-6, Page 16, FIG. 7.
According to a new trend, an additional energy store, for example a double-layer capacitor, is connected in series with the vehicle battery. An on-board electrical system of this type is known from the document Continental, ELKS 2008—“Elektrische Leistungsbordnetze und Komponenten von Straβenfahrzeugen”, contributions to the first Symposium with the same name held from 8 to 9 Oct. 2008, TU Braunschweig, ISBN: 978-3-937655-17-8, Page 90. In this on-board electrical system, the voltage drop during an engine start is compensated by connecting a double-layer capacitor in series with the vehicle battery. With this series connection, a voltage can be supplied to the load—in an ideal situation where the starter is not operated—which is greater than the battery voltage. This represents an undervoltage compensation. A vehicle battery and a double-layer capacitor which are connected in series are also disclosed in the published document DE 10 2005 042 154 A1.
The overall state-of-the-art is concerned with problems associated with compensating voltage drops in the on-board electrical system. It is a particular challenge to compensate not only the voltage drops, but to also compensate overvoltages in the on-board electrical system without adding substantial complexity and costs, namely without using an expensive DC voltage converter.
It is an object of the invention to provide a solution for reliably operating an electrical load in an on-board electrical system of the aforedescribed type without increasing its complexity.